Various types of operating circuits are known to start and operate compact fluorescent lamps. One type of circuit is illustrated in the FIG. 7 schematic of French Publication No. 0346782. This schematic is similar, in general principle, to the state of the art as practised in a lamp sold by the Osram Company under the registered trademark "DULUX EL" or in the lamp of the Philips Company which bears the denomination "PLC 20 Electronic". Using the circuit of French Publication 0346782 as an example of such circuits, after the two input terminals of the DC/AC converter (or oscillator) are energized by a DC voltage which appears across a filter capacitor, the starting capacitor C5 charges through a starting resistor R3 to a voltage which is substantially equal to the threshold voltage of the threshold element (i.e., the diac). The threshold element breaks down and supplies a pulse to the base terminal of transistor T2. As a result, transistor T2 begins to conduct. A current flows through transistor T2 and the load circuit. Subsequently, this transistor becomes non-conducting and the other transistor T1 becomes conducting. This process is then continuously repeated. This leads to an oscillation, i.e., an alternating current through the load circuit which includes the discharge tube.
It has been discovered that disadvantages may appear in some circuits similar to those described above. For example, when power to the circuit is removed, a momentary blink or flicker in the lamp may occur immediately after the tube is extinguished. It has been observed that when AC power to the circuit is removed, a voltage initially remains on the filter capacitor of the DC power supply. This filter capacitor voltage gradually depletes to a point (usually greater than the starter threshold voltage) where the oscillator shuts down. However, the starting capacitor is allowed to recharge to a point where the threshold element of the starting circuit triggers causing the oscillator to conduct for a short period of time. Consequently, the discharge tube will blink or flicker as a result of current from the filter capacitor flowing through the conducting transistors and load circuit. This conduction continues for approximately 10 msecs. until the filter capacitor voltage is less than the starter's trigger voltage.
Another disadvantage may appear in circuits similar to those described above at the end of lamp life when the emissive material on one or both of the filament electrodes has depleted. Although a discharge is unable to be established between the lamp electrodes, the oscillator may continue to conduct current through circuit components causing an unnecessary consumption of power until, for example, the AC power source is disconnected or the lamp and tank capacitor are removed from the load circuit. In the instances where the lamp is permanently connected, such as in an integral lamp unit, this latter option is unavailable.